It is known to use documents having such position identification markings in combination with a pen having an imaging system, such as a CMOS camera, within it, which is arranged to image a small area of the page close to the pen nib, which is illuminated by an infra red LED. The IR wavelength is chosen as it is reflected by carbon based black ink. The pen includes a processor having image processing capabilities and a memory and is triggered by a force sensor in the nib to record images from the camera as the pen is moved across the document. From these images the pen can determine the position of any marks made on the document by the pen. The pen markings can be stored directly as graphic images, which can then be stored and displayed in combination with other markings on the document. In some applications the simple recognition that a mark has been made by the pen on a predefined area of the document can be recorded, and this information used in any suitable way. This allows, for example, forms with check boxes on to be provided and the marking of the check boxes with the pen detected. In further applications the pen markings are analysed using character recognition tools and stored digitally as text. Systems using this technology are available from Anoto AB.
Many data encoding patterns are based on a grid structure, and it is important that the scale of the pattern is correct, so that the grid spacing conforms to the required specification. This ensures that the pattern can be read accurately by the reading device, and that, where the pattern is a position identifying pattern, the position indicated by the pattern is accurate. If offset printing is used then the pattern can be reproduced accurately and the grid spacing can be set to match the pattern specification. However, when the pattern is printed using more flexible and cheaper printers, such as a laser jet printer, the resolution of the printer may not enable it to print the marks in exactly the required position. Therefore when the printer is requested to print the marks in certain positions, the actual positions in which they are printed do not correspond exactly with the requested position, but are rounded to the closest position which is an integer multiple of the printer resolution. Typically, the error in the positions of the marks builds up across the document until it reaches about half of the printer resolution. Then the error is switched to be in the opposite direction, and from that point on it decreases across the page to zero, and then starts to build up again. The points at which the direction of the error is swapped produce visible bands on the document.